1. Field of the Invention
The invention relates to a method for purifying polycrystalline silicon with an improved flow of the purifying solutions in the process.
2. Description of the Related Art
High-purity semiconductor material is required for the production of solar cells or electronic components, such as memory elements or microprocessors for example. The dopants introduced in a targeted manner are the only impurities that a material of this type should have in the most expedient case. Therefore, endeavors are made to keep the concentrations of harmful impurities as low as possible. It is often observed that semiconductor material that has already been produced with high purity is contaminated again in the course of further processing to form the target products. Therefore, complicated purifying steps repeatedly become necessary in order to restore the original purity.
In particular, contamination by metal atoms should be regarded as critical since the latter can alter the electrical properties of the semiconductor material in a harmful manner. If the semiconductor material that is to be comminuted is comminuted, in the manner that has predominantly been customary heretofore, by means of mechanical tools, such as crushers made of steel, for example, then the fragments have to be subjected to surface purifying prior to melting.
In order to remove the impurities, by way of example, the surface of the mechanically processed polycrystalline silicon is etched using a mixture of nitric acid and hydrofluoric acid. In the course of the process, the metal particles are attacked to a great extent by the acid mixture during the preliminary purifying. Metal carbide residues remain, and are dissolved to the greatest possible extent during the HF/HNO3 main purifying.
In this case, the polysilicon fragments are usually dipped successively into different purifying solutions during purifying in baskets or basins.
EP 0905796 describes a purifying process comprising preliminary purifying by means of a mixture comprising HF/HCl/H2O2, main purifying by means of HF/HNO3 and subsequent hydrophilization of the silicon surface by means of HCl/H2O2. During the purifying process, rinsing takes place in throughflow or dump tanks. In this case, the silicon fragments are purified in a purifying machine on the basis of an up and down movement. In addition, the basin loaded with polysilicon fragments can also move completely out of the liquid during the lifting/lowering movement, in order that the purifying solution can completely drain away from the silicon fragments. A disadvantage that emerges is that spots having a gray appearance are found on the silicon fragments in the course of the process. Investigations have shown that the gray spots always occur at the contact points between individual poly fragments or between poly fragments and the process basin wall. The cause is excessively little flow in the dead water zones between the individual poly fragments. A further disadvantage in the method described is the undesirable residual acid concentration at the fragments. Despite greatly increasing the flow rate up to complete liquid exchange in the tank in less than one second, it has always been possible to detect tiny acid traces in the pptw range by means of ion chromatography and IC measurements on the purified poly fragments.
It is also known from the prior art to etch polysilicon rods (FZ rods) and thin rods in drum apparatuses. Although experiments on apparatuses of this type with poly fragments show that no superimposed spots arise at rotational speeds of greater than revolution per minute, the sharp-edged polysilicon fragments produce visible abrasion of the drum material even at a low rotational speed of the drum. This abrasion is unacceptable for subsequent application of the polysilicon fragments as semiconductor material.
US 2006/0042539 describes an apparatus in which the tray in the container is caused to effect a regulated translational movement in a lateral direction during the treatment duration. However, the translational movement leads to the same results as the lifting/lowering movement described in EP 0905 796. The problem of the dead water zone at the contact points cannot be solved with an apparatus of this type either.
DE 69905538 describes a centrifuge apparatus for purifying small parts. In this case, a rearrangement of the parts is achieved by means of a continual change between centrifugal force and gravitation. The centrifugal force takes effect at high rotational speed, and the gravitation at low rotational speed. The parts are continually rearranged as a result of this change. What is disadvantageous here, too, is that abrasion arises as a result of the relative movement with the vessel, the abrasion being unacceptable for semiconductor-conforming polysilicon fragments.
None of the solutions known from the prior art leads to useable results. If the etching devices are linked with rearrangement of the polysilicon fragments, unacceptable abrasion of the containers must always be reckoned with. Applications such as are known from the purifying and etching of semiconductor wafers cannot be employed owing to the different geometries of polysilicon fragments and wafers.